The present invention relates to an optical pickup of an optical disk apparatus and a servo method in a multibeam optical pickup.
Recently, a multibeam optical pickup has been practically used in an optical disk apparatus. In such a multibeam optical pickup, an optical path for lighting is constructed such that a plurality of beams from a semiconductor laser array can be guided onto an optical disk by a common optical path. One of the plural beams is used for record and another one of the plural beams is used for reproduction. At the time of the recording mode, while a record information is written by the recording beam, the confirmation and reproduction are performed by the reproducing beam. At the time of the reproducing mode, the record information is reproduced by the reproducing beam.
There are two modes of record and confirmation-reproduction with respect to the recording operation onto the optical disk so that the time required to perform the recording operation becomes long in the conventional single beam system. However, in the multibeam optical pickup, the writing operation and the confirming-reproducing operation are almost simultaneously performed so that the problems about the recording time can be effectively solved.
Such a multibeam optical pickup has the following problems.
At the time of the recording mode, no shift is caused with respect to the reproducing beam since each servo signal is provided from the reproducing beam. However, there is a possibility in which a shift is caused with respect to the recording beam so that the reliability of a written bit is reduced. Since the confirmation and reproduction are performed in the recording mode, it can be considered that the recording operation is suitably performed as long as the confirmation and reproduction are correctly performed. However, when the reproduction is performed in another optical pickup, problems about the reproduction of the record information can be caused by the shift of the recording beam.
In the optical disk apparatus, a beam from a semiconductor laser is focused onto a small spot by an objective lens and is irradiated onto the optical disk to record, reproduce or erase the information. Such an optical disk apparatus has the advantages that the optical disk has a large capacity and the cost per bit is cheap and the optical disk is removable, thereby facilitating the treatment thereof.
However, this optical disk apparatus has the disadvantages that the processing time is longer than that in hard disk such as a magnetic disk, etc. Namely, when the recording operation is performed by the optical disk and a command is given, an accessing operation is first performed and a small spot is moved to a desired address position to perform the recording operation. After the recording operation has been completed, the reproducing operation is performed to confirm a written information. The processing time becomes long by such a construction. In the optical disk apparatus, the recording mode of the optical disk is constituted by the recording operation and the confirming-reproducing operation.
To solve such problems about the processing time, the optical pickup of a multibeam system has been proposed. In this system, a plurality of laser beams are converged onto the same track of the optical disk. almost at the same time when the recording operation is performed by a preceding beam, the reproducing operation for confirming the written information is performed by a subsequent beam. By such a system, both the recording operation and the reproducing operation for confirmation are almost simultaneously performed so that the time required in the recording mode can be reduced. The optical pickup of such multibeam system is disclosed in Japanese published patent No. 57-60697 for example.
In the pickup for performing the recording operation and the confirming-reproducing operation by two beams, a servo control is performed by providing a servo signal for track/focus from the subsequent reproducing beam. The reason is that, since light is emitted by a constant power on the reproducing beam side, the operation is considered to be simpler and more stable than that for providing the servo signal based on a modulated light such as the recording beam.
However, in such a pickup, since the servo signal is provided by confirming-reproducing beam B at the recording time, a track shift can be caused with respect to recording beam A although no track shift is caused with respect to confirming-reproducing beam B. The reason is that, even when reproducing beam B is located in a correct position with respect to the track, recording beam A is not always located in a correct position with respect to the track. As a result, the track shift with respect to the pit recorded by recording beam A can be caused so that a reliable recording operation cannot be performed.
Further, the information recording medium judged to be recorded in such a state of the track shift and a focus shift and correctly recorded by the confirming-reproducing operation, tends to cause errors in operation when the shift of the reproducing beam is reproduced in another drive different from the drive in which the shift is recorded. Therefore, the reliability of the information recording medium is reduced.
Further, the track shift is caused with respect to the recording beam and the reliability of the recording operation is reduced when the information recording medium is eccentric. The track shift can be caused by the difference in curvature between the inner and outer circumferences of the track, a disturbance of the track, the eccentricity of the optical disk, etc.
There is a method for providing a signal for the focus/track servo control based on preceding beam A for record instead of subsequent beam B for reproduction. However, even in such a method, the focus or track shift is caused with respect to the subsequent beam for confirmation and reproduction. Therefore, even when the tracking servo control is performed by the beam for record to correctly perform the tracking operation, there is a case in which the track shift is caused with respect to the confirming-reproducing beam by the eccentricity of the recording medium.